Good morning! New member, thanks for letting us join! We are a small cafe in Central Washington State, we have an older 4 head La Marzocco Linea, it's been refurbished and running great for the past 2 years. This morning only the 2 right side heads are working, the 2 left side heads won't heat up, any thoughts? Thanks!

How are you measuring "power" at the thermostat? Do you mean no voltage across it measured with a voltmeter? Or no power measured with an ampclamp? Or are you going by the fact that the heads aren't hot and suspecting a thermostat?

I strongly recommend against testing a live machine unless you know exactly what you're doing and how the tools should be used. That said, if you can give us a description of the symptoms and the test results, we can probably narrow it down.

Lastly, you should probably set up a service call with your local La Marzocco service professional.

The 4 group has 3 different boiler, 2 for 2 groups each and the "big" steam one,
check the resettable thermostats (one on each head which will be a little odd that they both go out at once) or the BREW thermostat on the front right boiler,
if it is the old style,,,,,
get a hold (if you like to do so of course) of us for alternative to the very very expensive thermostat no longer available.
Stefano